1. Field of the Invention
The present invention relates to a CVD (Chemical Vapor Deposition) apparatus for Cu formation.
2. Description of the Background Art
Conventionally, material containing Al with Cu added thereto has been used as an interconnection material for LSIs. Interconnection formed of pure Al has short electromigration lifetime, resulting in unsatisfactory reliability. Therefore, Cu is added to Al to improve reliability. General concentration of Cu is about 0.5 wt %. This value is selected since, with this concentration, electromigration lifetime is significantly improved, increase in resistivity is relatively small and Cu residue is not observed in processing utilizing Reactive Ion Etching (RIE).
It has been generally considered that electromigration lifetime is inversely proportional to the square of current density. As the degree of integration of LSIs has been ever increasing, cross section of a line or interconnection has been decreased, and current density has been increased. Therefore, there has been ever increasing demand for higher electromigration resistance. Further, since the cross section of a line decreases as the degree of integration of LSIs has been increasing, line resistance and line RC delay are also increased. As a result, it is expected that line RC delay will affect speed of operation of the devices of a generation having the line width of 0.15 .mu.m or smaller. It has been increasingly difficult for currently used AlCu alloy to solve these problems. Cu has as low a resistivity as about 70% that of pure Al, and has electromigration lifetime longer by about three orders than AlCu. Therefore, Cu is considered a promising material for interconnections for the generation of 0.15 .mu.m or smaller.
In the Cu interconnection process, processing or treatment of Cu has been an essential problem, since conventional RIE is difficult as vapor pressure of Cu chloride is extremely low. Therefore, a process called Damascene process is adopted for forming Cu interconnection in place of the process employing RIE. The Damascene process is described, for example, in "Interconnection Process Employing Damascene Method" published in monthly magazine Semiconductor World, December, 1995.
In the Damascene process, a trench is formed in an insulating film, the trench is filled by a metal (Cu), and the metal (Cu) formed on portions other than the trench is removed, for example, by Chemical Mechanical Polishing (CMP). At this time, it is necessary to fill the trench fully with Cu. Conventional sputtering cannot attain sufficient filling, thereby causing disconnection or unsatisfactory reliability. In order to fill the trench with Cu, a CVD method having superior filling characteristic is necessary.
Characteristic required of a CVD raw material includes that the material has high vapor pressure, allows deposition of highly pure metal Cu and that the material is liquid or gas. Very few material satisfy such conditions. Generally, Cu(HFA) with adducted organic molecules is used where HFA means Hexafluoroacetylacetonate. A typical example includes Cu(HFA)(TMVS), or Hexafluoroacetylacetonate Copper Trimethylvinyl Silane adduct, which is expressed by the following molecular expression (1) and has molecular structure such as shown in FIG. 6. EQU Cu[(CF.sub.3 CO).sub.2 CH].CH.sub.2 .dbd.CHSi(CH.sub.3).sub.3(1)
Cu (HFA)(TMVS) is an organic material having relatively high vapor pressure and is in liquid phase at a normal temperature. Reaction proceeds in accordance with disproportination as represented by the following expression (2), and hence a film having high purity is obtained. EQU 2Cu(HFA)(TMVS).fwdarw..rarw.Cu+Cu(HFA).sub.2 +2TMVS (2)
Since Cu(HFA)(TMVS) is liquid at a normal temperature, bubbling or direct liquid injection is employed for raw material supply. In bubbling method, the liquid is vaporized by heating a source container. A carrier gas is introduced to the source container, and the vaporized raw material is introduced to a chamber. The supply of the raw material is determined by the heating temperature of the source container and a flow rate of the carrier gas. In the direct liquid injection method, the flow rate of the liquid raw material is controlled by a liquid mass flow controller or by a pump, and the raw material is guided to a vaporizer. The raw material is vaporized in the vaporizer (which heats in vacuum), mixed with the carrier gas, and then introduced to the chamber.
By employing the method described above, it is possible to exactly control the flow rate. Further, unlike bubbling, it is not necessary to heat the source container, and hence degradation of the raw material caused by heat can be avoided. In either of the supplying methods described above, portions of delivery system, valve, shower head and so on through which the vaporized raw material passes are kept at approximately the same temperature as the evaporating temperature, so as to prevent liquefaction of the raw material in gas phase. Further, walls of the chamber, exhaust lines and an exhaust pump are heated to prevent liquefaction of the raw material and deposition of resulting substances such as Cu(HFA).sub.2 or TMVS. Generally, SUS (stainless steel) is used for the source container, liquid mass flow controller, pump, vaporizer, delivery system, valves and shower head.
However, Cu deposition reaction quickly proceeds when the raw material such as described above is employed, and the reaction is initiated simply by the presence of the raw material. Therefore, there arises a problem that a substance resulting from the reaction, such as Cu or Cu(HFA).sub.2 tends to be deposited on the surfaces of the delivery system and the chamber. Such substance resulting from the reaction may cause particles, deteriorating production yield of interconnections and making shorter the period for maintenance.